A new insight into ethoxyquin fate in surface waters: Stability, direct and indirect photochemical behaviour and the identification of main products

Article abstract of DOI:10.1016/j.jphotochem.2015.06.030

In this work, the fate of ethoxyquin (ETX), an antioxidant used as a food preservative and a pesticide, is investigated under ultraviolet and visible irradiation. The fast photolysis observed under environmentally closed conditions results in estimated degradation rates (RETXd) of 1.6 × 10^-8 and 8.7 × 10^-7 M s^-1 at pH 2.7 and 8.0, respectively. Under our experimental conditions, the polychromatic quantum yield (R^d_ETX) is evaluated to be 5.0 × 10^-3 and 1.4 × 10^-1 for the protonated and anionic forms of ETX, respectively. The effect of hydroxyl radical on ETX degradation is evaluated, and the degradation kinetics profiles suggest that transformation via hydroxyl radical can be neglected compared with direct photolysis. Structures for the main degradation products (hydrolysis, photochemical and HO?- mediated derivative) are proposed. The kinetic parameters are used as inputs in APEX software to model the degradation of ETX as a function of different scenarios in surface waters. Finally, the irradiation of ETX under sun-simulated conditions estimates the life of this compound to be ～38 s.

Full text of DOI:10.1016/j.jphotochem.2015.06.030

Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A new insight into ethoxyquin fate in surface waters: Stability, direct
and indirect photochemical behaviour and the identification of main
products
Aziza Toure Bintoua,b, Angelica Biancoa,b, Gilles Mailhota,b, Marcello Brigantea,b,
*
a
Université Clermont Auvergne, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France
b
CNRS, UMR 6296, ICCF, F-63171 Aubiere, France
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 7 April 2015
Received in revised form 23 June 2015
Accepted 24 June 2015
Available online 27 June 2015
In this work, the fate of ethoxyquin (ETX), an antioxidant used as a food preservative and a pesticide, is
investigated under ultraviolet and visible irradiation. The fast photolysis observed under environmen-
tally closed conditions results in estimated degradation rates ðRdETXÞ of 1.6 ꢀ10ꢁ8 and 8.7 ꢀ10ꢁ7 M sꢁ1 at
pH 2.7 and 8.0, respectively. Under our experimental conditions, the polychromatic quantum yield (
’
)
ETX
is evaluated to be 5.0 ꢀ10ꢁ3 and 1.4 ꢀ10ꢁ1 for the protonated and anionic forms of ETX, respectively. The
effect of hydroxyl radical on ETX degradation is evaluated, and the degradation kinetics profiles suggest
that transformation via hydroxyl radical can be neglected compared with direct photolysis. Structures for
the main degradation products (hydrolysis, photochemical and HOꢂ- mediated derivative) are proposed.
The kinetic parameters are used as inputs in APEX software to model the degradation of ETX as a function
of different scenarios in surface waters. Finally, the irradiation of ETX under sun-simulated conditions
estimates the life of this compound to be ꢃ38 s.
Keywords:
Environmental fate
Pollutant photochemistry
Surface water transformation
Polychromatic quantum yield
Pollutant lifetime
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
additive in U.S.A. An ETX risk assessment was performed using a
streamlined process for lower risk/exposure pesticide chemicals.
The investigation of xenobiotic fate has received increasing
attention in the last two decades due to their quantification in
wastewaters and aquatic compartments [1–3]. Sun radiation is
now considered as a key factor responsible for the transformation
of pesticides and drugs via direct photolysis or the formation of
transient species in surface waters [4,5].
Ethoxyquin (ETX) is an antioxidant used as a food preservative
and a pesticide [6]. ETX is one of the most used antioxidants
because it prevents the rancidification of fats in animal feed,
especially at low temperatures, where the rate of decomposition of
this product is low because the rate constant for the interaction of
peroxy-radicals with ethoxyquin is significantly greater than the
corresponding constant for fatty acids and because the product
formed is relatively inactive [7]. ETX has been detected in paprika
[8] and pears and apples [9]. Its main role is to prevent colour loss
due to oxidation of the natural carotenoid pigments and to control
scald (browning). ETX is not authorised in food production in
Australia or within the European Union, but it is an accepted
Although the ETX toxicology database is not complete, it provides
adequate information for evaluating and characterising the risks
under the FIFRA (Federal Insecticide, Fungicide and Rodenticide
Act) and FQPA (Food Quality protection Act), which were both
released by the EPA (Environmental protection Agency) to limit the
use of this chemical. ETX has low to moderate acute toxicity by oral
(Category III), dermal (Category III) and inhalation (Category III)
exposure routes. It is not an eye irritant (Category IV), and it
produces minimal irritation to the skin (Category IV) [10]. Tests in
animals show that ETX has a weak sensitizing potential; however,
extensive human experience in using this chemical has indicated
contact dermatitis, thus necessitating the discontinuation of
working with ETX [10]. Although it has been approved for use
in foods and as a spray insecticide for fruits, ETX has not been
thoroughly tested for carcinogenic potential. ETX has been
suggested that as a possible carcinogen because a very closely
related chemical, 1,2-dihydro-2,2,4-trimethylquinoline, has been
demonstrated to be carcinogenic in rats [11]. Although ETX was
assessed to be slightly toxic to fish, it is commonly used as
antioxidant in fish meal and fish oil to limit lipid oxidation. The
highest levels of ETX (0.17 mg kgꢁ1) were found in farmed Atlantic
salmon fillets [12], while the lowest concentrations of the
* Corresponding author.
E-mail address: marcello.brigante@univ-bpclermont.fr (M. Brigante).
http://dx.doi.org/10.1016/j.jphotochem.2015.06.030
1010-6030/ã 2015 Elsevier B.V. All rights reserved.

A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
119
synthetic antioxidant found were in cod. The concentration of the
2.3. Irradiation procedures
oxidation product ethoxyquin dimer (EQDM) was more than ten
times higher than the concentration of parent ETX in Atlantic
salmon, whereas this dimer was not detected in cod fillets [13]. The
reactivity of ETX in natural waters is completely unknown. Despite
its significant use in aquaculture, the environmental fate of ETX in
aquatic environments has not been studied. In the present work,
the transformation of ETX in water is investigated under sun-
simulated conditions. The kinetic parameters obtained from
steady-state irradiation are used to assess the life time of ETX
under different physico-chemical conditions such as irradiation
energy, hydroxyl radical concentration, water column depth and
presence of organic matter.
Irradiation was performed using a homemade irradiation
system placed in a cylindrical stainless-steel container equipped
with six fluorescent lamps (Philips TL D15W/05) with emission
spectra ranging from 300 to 500 nm. The photoreactor, a Pyrex
tube with an internal diameter of 2.6 cm, was placed in the centre
of the setup. The total solution volume was 50 mL, and all
experiments were carried out at room temperature (293 ꢅ 2 K).
Samples were taken from the reaction tube at fixed interval times.
The emission spectrum of the irradiation system with two lamps
(Fig. 1) was recorded using fibre optics coupled with a charge-
coupled device (CCD) spectrophotometer (Ocean Optics USD
2000 + UV–vis) and normalised using a PNA/pyridine actinometer
method [14]. The photon flux reaching the solution (Ia) was
estimated using Eq. (1):
2. Material and methods
2.1. Reagents
D
fPNA
NPNA
Ia ¼
(1)
ETX was purchased from Fluka (ꢄ97.8%) and used as received.
Analytical grade perchloric acid, sodium hydroxide, sodium nitrite
were purchased from Prolabo. Terephthalic acid (>99%) was
purchased from Alfa Aesar. HPLC grade acetonitrile was obtained
from VWR Prolabo. Ammonium acetate (ꢄ 97%) was purchased
from Fluka.
where
D
NPNA represents the amount of PNA decomposed and fPNA
,
the photodegradation quantum yield, is calculated using the
relation
f
PNA = 0.44 ꢀ [pyridine] + 0.00028. Using 0.1 mM of pyri-
dine fPNA was calculated to be 3.24 ꢀ10ꢁ4
The total irradiance between 300 and 500 nm reaching the
solution was then estimated to be between 3.5 and 20.8 W cmꢁ2
(corresponding to 0.67 and 4.01 ꢀ1019 photons cmꢁ2 sꢁ1) using one
and six lamps, respectively.
All solutions were prepared with distilled water purified by a
Millipore system with a resistivity of 18 M
Vcm and DOC < 0.1 mg
Lꢁ1
.
Solutions were doped with hydrogen peroxide (H2O2) and
nitrate (NO3ꢁ) in order to generate hydroxyl radical under
irradiation. Both compounds are chosen because they are naturally
occurring compounds generally found in surface and/or atmo-
spheric waters and due to low reactivity with photogenerated
hydroxyl radical (i.e. generally not considered as HOꢂ scavengers).
2.2. Solution preparation
ETX is weakly soluble (170 mg Lꢁ1) [16] in water and the UV–vis
spectrum of ETX solution (1 ꢀ10ꢁ4 M) shows a time evolution if
kept in the dark at room temperature (Fig. SM1). Therefore stability
of ETX in acetonitrile was tested and any notably modification was
observed after 1 week in dark. For these reasons, a concentrated
2.4. UV–vis and fluorescence spectroscopy
stock solution (2 ꢀ10ꢁ3 M) in acetonitrile (solubility of 462 g Lꢁ1
)
was stored in the dark, and fresh solutions were prepared just
before irradiation by diluting aliquots in Milli-Q water (approxi-
mately 2% acetonitrile by volume) to obtain the desired
concentration (5 ꢀ10ꢁ5 M).
Solution absorbance was determined using a Varian Cary
300 Scan UV–vis double beam spectrophotometer, while hydroxyl
radical formation rate was determined using a terephthalic acid
chemical probe by the method previously described by Charbouil-
lot et al. [15]. The formation of hydroxyterephthalic acid was
quantified using a Varian Cary Eclipse fluorescence spectropho-
tometer with an excitation wavelength (lex) of 320 nm and an
Fig. 1. UV–vis spectra of ETX at pH 2.7 (protonated form) and pH 8.0 (anionic form) and emission spectrum for the irradiation system with two lamps.

120
A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
Fig. 2. Evolution of the ethoxyquin UV–vis spectrum as a function of pH. Inset: chemical structure of ETX and Abs at 343 nm as a function of pH.
emission wavelength (lem) of 420 nm. The scan rate and bandpass
were 600 nm minꢁ1 and 10 nm, respectively, for both excitation
and emission.
equipped with an ESI ion source (positive ion mode) and TOF and
UV detectors. The adopted column was a Sphere Clone C18
(Phenomenex, 4.6 ꢀ 250 mm ꢀ 5
m
m), and the gradient elution was
; a
as follows: initially 5% acetonitrile and 95% water-formic acid 1
m
2.5. Liquid chromatography
linear gradient to 95% acetonitrile within 22 min; constant
conditions for 20 min. The flow rate was 0.4 mL minꢁ1, and the
UV detector was set at 254 nm.
ETX concentration was determined using a Water Acquity Ultra
Performance LC instrument equipped with a Water Acquity Photo
Diode Array detector and fluorescence detector. Compound
separation was performed using a Water Acquity UPLC C18
2.6. Kinetic treatment of the data
(100 mm ꢀ 2.1 mm ꢀ 1.7
0.01 M ammonium buffer and 60% acetonitrile) and an eluent flow
rate of 0.3 mL minꢁ1
The ETX retention time under these
conditions was 3.0 min. UV–vis detection was performed at
m
M) column with isocratic elution (40%
The initial rates of ETX degradations were determined by fitting
the experimental data with the pseudo-first order decay equation
of the form ½ETXꢆt ¼ ½ETXꢆ0expðꢁkdETXtÞ, where ½ETXꢆ0 and ½ETXꢆt are
.
the concentrations of ETX at the initial time at time t, respectively,
l
= 223 nm.
and kdETX is the pseudo-first decay constant. The degradation of ETX
(RdETX, M sꢁ1) could then be obtained from kdETX ꢀ ½ETXꢆ0.
The chemical structures of ETX transformation products were
proposed based on HPLC (Agilent 1100 Series, binary pump)
Fig. 3. Correlation between ETX degradation rates and irradiation energy between 290 and 500 nm at pH 2.7 and 8.0. Dashed lines denote the 95% confidence of the liner fit.

A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
121
Table 1
Degradation rates of ETX ðRdETXÞ at pH 2.7 and 8.0 using different nitrite concentrations in the presence or absence of chloride. The errors are derived at the 1
s
level from the scattering of the data using an exponential decay fit.
pH 2.7
pH 8.0
RdETX (M s -1)
RdETX
(M s -1)
ETX photolyꢁsis
1.64 ꢅ 0.06 ꢀ10ꢁ8
2.25 ꢅ 0.03 ꢀ10ꢁ8
3.66 ꢅ 0.04 ꢀ10ꢁ8
5.20 ꢅ 0.03 ꢀ10ꢁ8
1.53 ꢅ 0.15 ꢀ10ꢁ7
5.78 ꢅ 0.25 ꢀ10ꢁ7
ETX photolyꢁsis
8.67 ꢅ 0.33 ꢀ10ꢁ7
1.05 ꢅ 0.03 ꢀ10ꢁ6
1.12 ꢅ 0.06 ꢀ10ꢁ6
10
20
30
10
20
m
M [NO2ꢁ
]
50
50
m
M [NO2 ]
m
m
m
M [NO2ꢁ
]
]
m
M [NO2ꢁ] + 0.5 M Clꢁ
M [NO2
M [NO2ꢁ] + 0.5 M Clꢁ
m
M [NO2ꢁ] + 0.5 M Clꢁ
Fig. 4. Concentration evolution vs. irradiation time of ETX at pH 2.7 and 8.0 under direct photolysis and in the presence of nitrite and chloride. Solid lines are the exponential
decay fits of the experimental data.

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A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
RdETX
Ia
ETX
300ꢁ500nm
Half-life times ðt1 Þ are evaluated considering the first order
=
f
¼
(3)
2
rate constant ðkdETXÞ of ethoxyquin and using Eq. (2)
lnð2Þ
where RdETX is the ETX degradation rate (M sꢁ1) and Ia is the
absorbed photon flux per unit of surface and unit of time. The latter
was calculated from Eq. (4):
t1=2
¼
(2)
kdETX
Z
e l
ð
Ia
¼
I0ðl
Þð1 ꢁ 10ꢁ Þd½ETXꢆÞ
(4)
To compare the photodegradation efficiency at pH 2, 7 and 8.0,
the polychromatic quantum yield between 300 and 500 nm was
l
ETX
ETX
where I0 is the incident photon flux,
coefficient of ETX, d the optical path length inside the cells and
[ETX] the initial ETX concentration.
e
the molar absorption
calculated ð
f
Þ.
f
is defined as the ratio between
300ꢁ500nm
300ꢁ500nm
the numbers of transformed molecules and the numbers of
photons absorbed by solution (Eq. (3)):
Table 2
Degradation product identification for ETX under direct (photolysis) and indirect (HOꢂ-mediated) photochemical conditions. Rt denotes the retention time using the HPLC
conditions reported in the materials and methods section. [M + H]+ is the HPLC-MS molecular peak for the reported irradiation conditions under which the molecule was
detected.
Rt (min)
[M + H]+
218
Fragments
Conditions
Structure
6.7
ETX
8.6
203
178, 167,138
Hydrolysis and direct photolysis
9.5
220
202, 173, 167
Direct photolysis
12.2
233
173
Hydrolysis, direct photolysis,
13.4
252
236, 214
Indirect Photolysis hydroxyl radical

A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
123
Table 2 (Continued)
Rt (min)
[M + H]+
433
Fragments
Conditions
Structure
21.6
Direct photolysis
22
448
Direct photolysis
3. Results and discussion
400 nm) than the acidic form (200–320 nm), as seen in Fig.1, which
shows the spectral overlap with the spectral emissions of the
adopted lamps. ETX presents two absorption maxima at 223 nm
and 343 nm.
3.1. Kinetic study
ETX is weakly soluble (170 mg Lꢁ1) [16] in water, and a fresh
1 ꢀ10ꢁ4 M solution of ETX kept in the dark at room temperature
shows a notably modified UV–vis spectrum (Fig. SM1). For these
Before irradiation experiments, the stability of ETX was tested
in the dark; degradation due to hydrolysis and thermal effects is
negligible during the adopted irradiation time (i.e. a few minutes).
ETX was irradiated at pH 2.7 (protonated form) and pH 8.0
(ionic form) using different numbers of lamps (corresponding to
different irradiation energies; Fig. 3). A linear correlation is found
reasons,
a
stock 2 ꢀ10ꢁ3
M solution of ETX in acetonitrile
(solubility of 462 g Lꢁ1) was stored in the dark, and fresh solutions
were prepared just before irradiation by diluting aliquots in Milli-Q
water (approximately 2% acetonitrile by volume) to obtain a
5 ꢀ10ꢁ5 M solution. Fig. 2 shows the absorption spectra of
5 ꢀ10ꢁ5 M aqueous solutions of ETX at different pH values. The
between lamp energy (LE, W mꢁ2) and ETX degradation rate ðRdETXÞ.
The RdETX values were 3.07 ꢅ 0.25 ꢀ10ꢁ9 LE and 2.78 ꢅ 0.20 ꢀ10ꢁ7 LE
for pH 2.7 and 8.0, respectively.
inset in Fig. 2 shows the variation in absorbance at 343 nm (Abs343
)
The polychromatic degradation quantum yield of ETX between
as a function of pH. The Abs343 increases as pH increases from 3 to
5 and then reaches a plateau at higher pH values. An inflection
point corresponding to the pKa of the molecules can be observed
around pH 4.5, in agreement with the literature data [17]. The
deprotonated form of ETX absorbs in a much wider domain (200–
ETX
300 and 500 nm ð
f
Þ can be estimated to be 5.03 ꢅ 0.03
300ꢁ500nm
ꢀ 10ꢁ3 and 1.40 ꢅ 0.19 ꢀ10ꢁ1 at pH 2.7 and 8.0, respectively. Under
the adopted irradiation conditions, the corresponding half-life
time ðt1 Þ can be evaluated as 30.1 min and 38 s at pH 2.7 and 8.0,
=
2

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A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
Fig. 5. Modelled fraction of ETX degradation attributed to direct photolysis and hydroxyl radical reactivity as a function of nitrite concentration and water column depth at pH
2.7 and 8.0.
(Cl2ꢂꢁ) (kCl
nitrite (kNO
= 3.0 ꢀ109 Mꢁ1 sꢁ1; R3) which is also scavenged by
ꢁ;HOꢂ
respectively. At pH 2.7, the degradation rate of ETX under
8
= 2.5 ꢀ10 Mꢁ1 sꢁ1; R4), but with a much smaller
photolysis RdETX is 1.64 ꢅ 0.06 ꢀ10ꢁ8 M sꢁ1; the degradation rate
ꢁ
2 ;Clꢂ
ꢁ
2
is enhanced two-fold with the addition of 20 m
M of NO2ꢁ (Table 1)
(around two orders of magnitude) rate constant compared with
the scavenging of HOꢂ.
due to the formation of hydroxyl radical (R1). In the presence of
chloride, ETX is completely degraded after ꢃ250 s (Fig. 4).
At pH 8.0, degradation under direct photolysis occurs much
faster, but no significant differences are measured in the presence
of nitrite and chloride, suggesting that the degradation of the
anionic form of ETX can be ascribed exclusively to direct
phototransformation.
þþClꢁ
Clꢁ þ HOꢂ H
!
!! Clꢂꢁ þ HOꢁ
(R3)
2
Clꢂꢁ þ NO2ꢁ ! 2Clꢁ þ ꢂNO2
(R4)
2
d;ETX
Considering 20 mM of nitrite as a hydroxyl radical source and R
NOꢁ2
This reactivity in acidic conditions can be explained considering
that nitrite is a photochemical source of hydroxyl radical (R1), and
Rd;ETX , the ETX degradation rates in the absence and presence of
NOꢁ2 þClꢁ
0.5 M of chloride and the fraction of HOꢂ reacting with nitrite can
be evaluated by the following equation:
ꢁ
2 ;HOꢂ
nitrite acts as a scavenger hydroxyl radical (kNO
Mꢁ1 sꢁ1; R2) via electron transfer reaction.
= 1.2 ꢀ1010
h
n
d;ETX
NOꢁ2 þ Hþ! ꢂNO þ HOꢂ
(R1)
R
NOꢁ2 þClꢁ
HOꢁꢂ
NO2
f
¼ 1 ꢁ
(5)
d;ETX
R
NOꢁ2
h
n
NOꢁ2 þ HOꢂ ! ꢂ NO2 þ HOꢁ
(R2)
In the presence of chloride in acidic conditions, photogenerated
hydroxyl radical leads to the generation of dichloride radical anion

A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
125
formation of this product is attributed to the generation of N-
radicals and the subsequent loss of methyl radical.
The main pathways for hydrolysis and photolysis are demeth-
ylation, with [M + H]+ = 202, and methylation, with [M + H]+ = 233.
The concentrations of these products increase with irradiation
time. Dimer production, which is known to occur during
photolysis, is observed with [M + H]+ = 433 at a retention time of
21.6 min [19].
The degradation of ETX was also performed in the presence of
H2O2 as a photochemical source of HOꢂ. Thus, the ETX solution was
irradiated in the presence of 2 ꢀ10ꢁ3 M hydrogen peroxide at pH
8.0 and analysed by mass spectrometry after 20 min of irradiation.
Under such conditions, only dihydroxylated ETX products are
found, with [M + H]+ of 252.
3.3. Modelling fate in natural waters
The photoreactivity of ETX in surface waters upon irradiation
(photolysis) and reaction with hydroxyl radicals was modelled by
using the savetable function of APEX [20].
The fractions of ETX transformation due to photolysis and
hydroxyl radical-mediated transformation was estimated at pH
2.7 and 8.0 (Fig. 5). Under both pH conditions, photolysis is the
main pathway of ETX degradation (Fig. 5A and B), and the
concentration of nitrite (a source of hydroxyl radical) and water
column depth have no significant effect on ETX disappearance. The
negligible influences of the presence of hydroxyl radical and water
column depth are confirmed in Fig. 5C and D. In fact, the main
photochemical sources of hydroxyl radicals in natural waters are
nitrate and nitrite. In this work, the reactivity of ETX with HOꢂ was
investigated using different concentrations of nitrite. ETX photo-
transformation by hydroxyl radical would plausibly be enhanced
by increasing the concentration of nitrite. Fig. 5C and 5D show the
fraction of ETX transformation attributed to hydroxyl radicals (on a
scale from 0 to 1), indicating that the contribution of hydroxyl
radical reactivity is negligible (<1%) for both the acidic and basic
forms of ETX.
The total half-life time of ETX can be evaluated considering that
the reverse of total half-life time is given by the sum of the reverse
half-life time for each reaction. Thus, ETX half-life time
ðt1=2ðETXÞtotÞ can be estimated using Eq. (7):
Fig. 6. Half-life time of ETX expressed as a fraction of solar sunny days (SSD) as a
function of DOM concentration and water column depth at pH 2.7 and 8.0.
1
1
1
¼
þ
(7)
t1=2ðETXÞtot t1=2ðETXÞHOꢂ t1=2ðETXÞh
n
Eq. (5) can also be expressed as:
where t1=2ðETXÞHOꢂ and t1=2ðETXÞh are the half-life times of ETX in
n
ꢀ
ꢁ
kNO
NOꢁ2
ꢁ
2 ;HOꢂ
HOꢁꢂ
NO2
the presence of hydroxyl radical and under photolysis, respectively.
Eq. (7) can be rearranged to give:
ꢀ
ꢁ
f
¼
(6)
kNO
NOꢁ2 þ kETX;HO ½ETXꢆ
ꢁ
ꢂ
2 ;HOꢂ
ꢁ
ꢂ
2 ;HOꢂ
where kNO
and kETX;HO are the second order rate constants of
t1=2ðETXÞ
ꢀ t1=2ðETXÞ
ꢂ
n
n
t1=2ðETXÞHO þ t1=2ðETXÞh
ꢀ
ꢁ
t1=2ðETXÞtot
¼
(8)
HOꢂ with nitrite and ETX, respectively, and NOꢁ2 and [ETX] are
their initial concentrations.
HOꢂ
h
ꢂ
Combining Eq. (5) with Eq. (6), we can estimate the kETX;HO to
Fig. 6 shows the ETX half-life time as a function of dissolved
organic matter (DOM) concentration (generally indicated in mg of
carbons per litres and indicated with DOC abbreviation) concen-
tration and the optical path length of sunlight in water, which can
be considered the main parameters influencing the half-life time of
organic compounds present in aquatic compartments. The
modelling study results are expressed as half-life times on
summer-sunny days (SSD) at pH 2.7 and 8.0. Two main effects
were modelled: the water column depth and the DOC concentra-
tion. High DOC values lead to three main effects: (i) high
scavenging of hydroxyl radicals; (ii) the production of reactive
species by excited state of organic matter; and (iii) a decrease in the
diffusion of light in the water column [21]. In the case of ETX, for
which the photolysis reaction is the most significant degradation
be 4.7 ꢀ108 Mꢁ1 sꢁ1 at pH 2.7. At pH 8.0, the hydroxyl radical
reaction with chloride does not lead to the formation of chloride
radical and dichloride radical anion [18]. Thus, as shown in Fig. 4,
the effect of OH-mediated ETX degradation can be considered to be
negligible compared with direct phototransformation.
3.2. Product identification
Different irradiated ETX solutions were analysed by HPLC-MS,
and the chemical structures of main transformation products are
proposed in Table 2.
6-Ethoxy-2,4-dimethylquinoline is quantified as a degradation
product in both hydroxlysis and photolysis processes. The

126
A.T. Bintou et al. / Journal of Photochemistry and Photobiology A: Chemistry 311 (2015) 118–126
pathway, the main effect of DOC is to decrease the diffusion of light
throughout the water column. At pH 2.7, the half-life time of ETX
ranges from 2 h at the water surface (with a negligible effect of
DOC) to 0.8 SSD. For the anionic form of ETX (pH 8.0), which is
expected to be the most relevant form in natural waters, the half-
life time ranges from 4 min (at the surface with low DOM
concentration) to 25 min (at a depth of 3 m with high DOM
concentration).
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In this work, we investigate for the first time, to the best of our
knowledge, the fate of ETX in natural waters considering the
photolysis process and HOꢂ-mediated transformation for the first
time. The deprotonated form of ETX, which is expected to be the
primary form in natural waters, presents a life-time of a few
minutes; thus, the quantification of this pesticide in natural waters
is quite difficult.
Irradiation products are identified, and photolysis quantum
yields are calculated under polychromatic irradiation (UVA and
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toxicity of photogenerated products, which are more capable of
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The authors acknowledge financial support from the “Fédéra-
tion des Recherches en Environnement” through the CPER
“Environnement” founded by the “Re’gion Auvergne,” the French
government and FEDER from the European Community.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
[19] http://www.fao.org/fileadmin/templates/agphome/documents/
jphotochem.2015.06.030.
Pests_Pesticides/JMPR/Evaluation99/14Ethoxyquin.pdf.in.
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a free software tool to predict the kinetics of
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